Frequency control system



Aug. 5, 1947. c. c. sToTz 2,425,013

:FREQUENCY CONTROL sYs'rm| Filed April 7, 1944 PULSED PULSE i TRANSMITTER cananxroa um: nrzuuxron 3| 29 CONTROL 4 t A.F.C. 1 LF'.

mxza AMPLIFIER FIG. 1. V 26 33 LOCAL OSCILLATOR 24 x 2s 27 2a SIGNAL. SIGNAL SIGNAL mxzn LF. UTILIZATION AMPLIFIER clncun IIIvr INVENTOR c RLST 2 BY M1 2 TTORNE 1 Patented Aug. 5, 1947 FREQUENCY CONTROL SYSTEM Carl C. Stotz, Garden City, N. Y., assignor to Sperry Gyroscope Company, Inc, a corporation of New York Application April 7, 1944, Serial No. 529,981

12 Claims. 1

The present invention relates to manual and automatic tuning of resonant circuits, and more particularly, to manual and automatic frequency control of tunable oscillators.

An object of the present invention is to provide improved methods and apparatus for tuning con trol of resonant circuits.

Another object of the present invention is to provide improved methods and apparatus for controlling the frequency of a superheterodyne high frequency oscillator.

A further object is to provide improved highgain amplifying means for signals derived from a frequency discriminator.

Another object is to provide an improved method and apparatus for sweeping the frequency of an oscillator through a tuning range in a predetermined manner, permitting transfer to automatic frequency control upon detection of a satisfactory oscillator frequency.

A further object is to provide an improved stable amplifier for the direct current component of a periodically varying signal.

Further objects will become apparent from the following description taken in conjunction with the drawings, of which:

Fig. 1 is a block diagram showing an embodiment of the present invention incorporated for illustrative purposes in a radio object detection system;

Fig. 2 is a schematic wiring diagram showing circuit details of certain parts of Fig. 1; and

Fig. 3 is a graphic representation of the operation of frequency sweep control apparatus included in the system of Fig. 2.

Similar characters of reference are used in Figs. 1 and 2 to indicate corresponding parts.

The invention will be described in an embodimerit especiall adapted for use with object detecting or locating devices, such as those including electromagnetic pulse transmitters and co operating superheterodyne receivers. However, it is to he understood that the invention is not restricted to use in such systems.

Fig. 1 shows a general block diagram of one orm of system including the present invention. In this figure, a pulse generator 2| of any conventional, type roduces a periodic pulse wave preferably having pulses of relatively short duralion, such as of the order of one-half to two microseconds. and with a pulse repetition frequency in the audio range, such as in the neighborhood of two thousand pulses per second. The output of generator 2! serves to control a pulsed high-frequency transmitter 22 feeding a transmitting antenna 23. Transmitter 22 may be of any suitable type adapted to produce an output carrier frequency preferably in the ultra high frequency range with a modulation envelope corresponding to the output of pulse generator 2|. In this way, energy radiated from antenna 23 will he a series of short pulses of ultra high frequency energy with a ulse repetition frequency determined by generator 2|.

The pulse waves radiated by antenna 23 will he reflected by any objects in its path, and a portion of the reflected energy will excite a receiving antenna 2&- feeding a suitable mixer 25 to which is also supplied a heterodyning or beating frequency wave from a local oscillator 26 as in standard superheterodyne receiver circuits. The output of mixer 25 then will have a frequency corresponding to the difference between the transmitted carrier frequency and that of oscillator 23, this difference frequency being known generally as an intermediate frequency. The intermediate frequency Wave output from mixer 25 may be suitably amplified in an intermediate frequency amplifier 21 and then be used to excite any suitable type of signal utilization circuit 28. The details of circuit 28 form no part of the present invention, but it may be generally noted that the intermediate frequency wave supplied to circult 28 may be utilized to indicate the position and/or movement of the remote reflecting object.

Since the received wave is generally of relatively low amplitude, requiring high amplification in the intermediate frequency amplifier 21, it will be appreciated that amplifier 21 will generally be of a highly selective, high-gain character usually referred to as a narrow pass-hand amplifier. In order to prevent the intermediate frequency output of mixer 25 from deviating from the passband of amplifier 21, which would cause markedly lowered gain and sensitivity, it is necessary to maintain local oscillator 26 at a substantially fixed frequency difference with respect to the transmitter carrier frequency.

According to the present invention, an improved form of automatic frequency control or frequency stabilized system is provided for controlling the local oscillator 26. For this purpose, in. broad terms, an intermediate frequency signal produced by heterodyning the outputs of transmitter 22 and oscillator 26 is applied to a frequency responsive circuit which controls the freouenoy of oscillator 26 to maintain the intermediate or difference frequency substantially constant. According to former practice, the difference frequency signal for automatic frequency control would be derived from the receiver mixer 25 or receiver intermediate frequency amplifier 21. However, this arrangement is subject to the distinct disadvantage that in the ultra high frequency range, where crystal mixers are used with frequencies of the order of magnitude of cycles per second, such crystal mixers become easily saturated by very high amplitude pulses or signals which may be received by the mixer.

In the arrangement shown, these harmful effects of crystal saturation are avoided by use of a separate mixer 29 for the frequency control system, independent from the signal mixer 25 used in the actual object-detecting circuit itself. As shown in Fig. 1, local oscillator 26 also supplies energy to automatic frequency control mixer 29, whose other input signal is supplied from the output of transmitter 22 through a suitable attenuator 31 of any desired type which will attenuate the transmitter output to a value incapable of producing saturation in the frequency control mixer crystal. The intermediate or difference frequency derived from mixer 29 is suitably amplified in an intermediate frequency amplifier 32 and used to actuate an automatic fre quency control circuit 33 which readjusts the output frequency of oscillator 26 to the desired value. Amplifier 32 is also controlled from the pulse generator 2| through a gate control circuit 34 for purposes later to be described.

The actual circuit arrangements of gate control 34, I. F. amplifier 32, A. F. C. circuit 33, and local oscillator 26 are shown in more detail in Fig. 2. In this figure, tube 35 is the last stage of amplifier 32, which may include entirely conventional preceding stages. if desired. As shown, tube 35 is of the pentode type and has a conventional circuit for amplifiers of its type, with the exception that the suppressor grid 36, instead of being connected directly to the cathode. is connected to be controlled by the gate control circuit 34, later to be described. For the present, it may be assumed that the potential supplied to suppressor grid 36 by control circuit 34 is substantially the potential of the cathode of tube 35,

permitting amplification of the intermediate frequency wave by the tube 35.

The output circuit of tube 35 includes a trans" former 31 having a primary 38 tuned to the desired difference between transmitter and local oscillator frequencies and a center-tapped secondary 3B. A coupling condenser 4| connects the intermediate-frequency high-potential terminal of primary 38 to the center tap 42 of the secondary 39. The other terminal of primary 38 may be connected through filter resistor 44 to the positive high-potential terminal 43 of a directcurrent source whose opposite, or negative terminal, is grounded. An intermediate-frequency by-pass condenser 45 provides a ground return for the intermediate frequency component of anode current of tube 35.

Secondary 39 may also be tuned to the desired intermediate frequency by an adjustable condenser 46. The tuned circuit 39, 46 may be provided with a broad band characteristic by a shunt resistor 41. An intermediate frequency choke coil 48 is also connected to center tap 42. A pair of rectifier tubes 49 and 5| have their anodes connected to respective outer terminals of secondary 39, their cathodes being connected to the other terminal of choke 48 through respective load resistors 52 and 53 shunted by respective intermediate-frequency by-pass condensers 54 and 55. The cathode of tube 5! is grounded at 56.

iiii

Rectifiers and 5| with their associated circuit elements form a conventional type of frequency discriminator and provide, between terminal 51 (connected to the cathode of tube 49) and ground, an output reversible polarity voltage corresponding in polarity and magnitude to the sense and amount of deviation of a prevailing or actual input intermediate frequency from a desired value corresponding to the resonant frequency of transformer 31.

Because of the pulsed character of the wave received by mixer 29 from transmitter 22 through attenuator 3!, the output potential derived between terminals 5! and 56 will be correspondingly pulsed and will therefore actually be a series of reversible-polarity potential pulses, the magnitude of the pulses corresponding to the amount of frequency deviation and the polarity of the pulses corresponding to the sense of frequency deviation of the local oscillator frequency from its desired value.

The alternating components of this pulsed frequency-responsive potential wave are amplified in a conventional resistance-capacity coupled alternating-current amplifier 58. The output of amplifier 58 is coupled to the input of a conventional phase inverter 59 which produces at the terminals 55 55', equal magnitude, opposite polarity potential pulses. These pulses are applied to the grids of balanced pulse detector tubes BI and 62 forming part of the present invention, through condensers 61 and II.

The anodes and cathodes of tubes GI and 52 are connected in a series circuit, comprising positive otential terminal 63 of a source of positive potential with respect to ground, the anode-cathode circuit of tube 6i, an adjustable voltage divider resistor 65, the anode-cathode circuit of tube 62, and a negative potential source 66, A potential varying substantially as the potential of terminal 66 is applied to the control grid of tube 5| through high-frequency coupling and direct-current blocking condenser 61 and a gridleak resistor 68, the cathode of tube 6| being connected to ground in an alternating-current sense by a by-pass condenser 69. Similarly, a potential varying substantially as the potential of the terminal is connected through a high-frequenc coupling and direct current blocking condenser i! to the control grid of tube 62, a similar grid-leak resistor 12 being provided and the cathode of tube 62 being by-passed to ground for alternating current by the condenser 13. The anode of tube 62 is also grounded for alternating current by condenser 14.

When the intermediate frequency output of amplifier 32 is at the desired frequency during the occurrence of the transmitted pulses, equal rectified currents flow through the frequency dis= criminator tubes 49 and 51. Accordingly, equal and opposite otentials are produced across condensers 54 and 55, with the result that terminal 51 is at ground potential. In this situation, no s gnal voltage is applied to the grid of amplifier tube 58, and the succeeding stages remain quiescent.

If, on the other hand, the signal delivered through I. F. amplifier 32 deviates from the desired frequency, unbalanced intermediate frequency voltages are applied to discriminator diodes 49 and 5! during the transmitted pulses, and a pulsating potential wave characterized by alternating components at the frequency corresponding to the pulse repetition rate and to harmonics thereof, is produced at terminal 51 with respect to ground. For frequency deviations in one direction, the potential of terminal 51 with respect to ground is positive during the detection of transmitted pulses, while for a frequency departure in the opposite sense, terminal 51 is negative with respect to ground during the brief transmitted pulses. Thus, the potential at terminal 51 may have a positive value during a short time interval such, for example, as one microsecond (during the ultra-high frequency transmission of transmitter 22, as determined by pulse generator 2|), thereafter remaining at substantially ground potential during a succeeding time interval such as approximately one millisecond (the interval between successive pulses) after which the cycle is repeated. Amplifier 5B, adapted to amplif only the alternating current components of the signal potential produced at terminal 51, accordingly applies to the grid of phase inverter 59 a signal which is strongly negative with respect to a normal (average) grid potential value during an interval of the order of one microsecond, after which the grid of tube 59 remains at a potential very slightly more positive than the normal quiescent grid potential during the interval between successive pulses.

Phase inverter 59, when provided as above described with pulsating potential signals, produces at terminals 60 and 60' rapid potential excursions of opposite polarities. Thus, during transmission of a ulse initiated by pulse generator 2!, terminals 60 and 50 are at greatly different potentials, one being greatly positive and the other equally negative with respect to the normal quiescent potentials at these points. During the intervals between the transmission pulses initiated by pulse generator 2 i, the potentials of ter minals 60 and 60 are only minutely different from their quiescent potential values, and with opposite directions of departure from those obtained during the transmission of the aforesaid pulses.

Due to the changes of potential of terminals 60 and 60', the grid-cathode circuits of detector tubes GI and 62, including condensers '61, 69 and H, 13, respectively, provide the diode detection which is characteristic of the input circuit of a grid-leak detector to which alternating current signals are applied. Thus, if the frequency departure of the signal introduced by amplifier 32 to transformer 31 is of such a sense as would produce a positive potential pulse at the grid of tube 6| during the transmission from pulsed transmitter 22, the grid-leak detector 6| will operate to produce an average grid potential negative with respect to the cathode, exceeding the average difference of potential of the grid of tube 62 with respect to the cathode of tube 62. Accordingly, the average cathode-anode resistance of tube 62 is lower than the cathode-anode resistance of tube 6!, and the direct-current potential at any point along potentiometer 65 is accordingly shifted in a negative direction with respect to the quiescent potential at that point. Conversely, if the output frequency of amplifier 32 is shifted in an opposite direction from the normal or desired frequency, the grids of tubes 6] and 52 will make opposite excursions during the operation of transmitter 22 under control of pulse generator 2i, and the direct-current potential at any point along potentiometer 65 will be shifted in a positive direction from its quiescent value. In this way, frequency shifts are converted into corresponding changes in potential at a point, such as at arm I5, of divider 55.

Capacitors 69, I3 and 14 are sufiiciently large to prevent reproduction at any point along potentiometer 65 of the pulsed wave characteristic of the signals applied to the grids of tubes 6i and 52. For any output frequency of intermediate frequency amplifier 32, potentiometer slider arm 15 may be adjusted to provide positive, negative, or zero potential with respect to the ground. The potential difi'erence between discriminator output terminal 51 and ground changes from a positive or a negative value during transmitted pulses resulting in intermediate frequency currents in amplifier 32 different from the desired intermediate frequency, to zero potential difference during the intervals between transmitted pulses. Thus, during a series of transmitted pulses characterized by a given discrepancy between the frequency of the intermediate frequency signal and the desired intermediate frequency, the time variations of the potential of terminal 51 with respect to ground may be analyzed into a direct component plus a series of alternating components.

According to former practice, the alternating components of potential of terminal 51 would be suppressed by a relatively large by-pass capacitor connected between this point and ground, and a direct-coupled amplifier would be used instead of the present alternating-current amplifier and detector system including vacuum tubes 58, 59, BI and 62. Direct coupled amplifiers suitable for high-gain amplification of a'direct potential usually are characterized by unstable performance and great difliculty of adjustment. The alternating-current amplifier and detector system described above overcomes these disadvantages, affording a highly amplified, stable reproduction at potentiometer arm 15 of the direct potential component at terminal 51 by the use of high-gain and stable amplifiers which amplify the alternating components of the signal in preference to the unidirectional component.

Slider arm 15 of potentiometer 65 is connected through resistor 16 to automatic frequency control terminal '11 of single-pole, double-throw switch 18. When the blade 19 of switch I8 is positioned for contact with terminal H, a version of the potential of potentiometer slider arm 15 is applied through a resistor to the grid of oscillator control tube 8!. This tube serves as a variable series resistance in the circuit supplying oscillator tube 82 from positive anode potential source 83. By virtue of the high resistance of resistor 80, the grid potential of tube 8| is confined to a range from very slightly positive values to strongly negative potential with respect to the cathode 84. Thus, the cathode-to-anode resistance of tube Bl may vary from the order of a hundred ohms to an almost infinite resistance, thus permitting wide-range control of the strength of oscillations generated by oscillator 82. The alternating current output of heater oscillator 82 is coupled through transformer 85 to a heater element 86 forming part of a variable separation strut used to control the spacing between flange 81 and flange 88 of velocity modulation tube 26. This oscillator is of the general type shown in Fig. 2 of Patent 2,250,511 to Varian et al., July 29, 1941, but in which the tuning is performed by varying the spacing between grids included in the resonator, as by varying the spacing between the rigid flanges 81, 88 connected to said grids.

Velocity modulation tube 26 forms the local oscillator of a superheterodyne receiver system, as shown in Fig. l, with output connector 89 available for connection to received signal mixer 25.

A similar output connector (not shown) may be provided in velocity modulation tube 26 for connection to the automatic frequency control mixer 29, from which intermediate frequency amplifier 32 receives its excitation.

An optional manual frequency control arrangement is provided by means of terminal 9| of switch 18, to which is connected the variable arm 92 of potentiometer 93. Potentiometer 93 is grounded at one end, being also connected to cathode 84 of variable resistance heater oscillator control tube 8 l. Potentiometer 93 is connected at the opposite end to the negative terminal 94 of a source of high potential whose other terminal (not shown) is grounded, so that when blade 19 of switch 18 is positioned in contact with terminal ill, the grid potential of tube 8| may be adjusted by means of manual control 92 completely independently of the operation of the automatic frequency control system.

The variable separation tuning strut 90 of the velocity modulation tube 25, shown as local oscillator for the superheterodyne receiver, operates to vary the volume of the resonant chamber 95 and so to increase the frequency of oscillation of tube 26 when the temperature of the tuning strut is increased by an increase of the heating power applied by oscillator 82 through transformer 85 to the heating element 86. Thus, if switch blade 19 is in position for manual tuning control, movement of potentiometer arm 92 along potentiometer 93 toward the grounded terminal decreases the resistance of series resistance tube 8!, permitting the amplitude of oscillation in heater oscillator 82 to increase. and thus to increase the temperature of the variable strut 90, increasing the frequency of oscillator 26.

During automatic frequency control operation of the system, with switch blade 79 positioned in contact with terminal l'l, any departure of the frequency of the signals amplified in amplifier 32 from a normal or desired frequency causes a change of the direct potential of slider arm 15, which in turn, varies the potential applied to the grid of series resistance tube BI, and so causes a temperature variation of oscillator 26 in a direction to restore the intermediate frequency signal to the proper value for normal operation In the event that when blade 19 of selector switch 18 is positioned for automatic frequency control, the frequency of oscillator 28 is greatly different from the desired frequency, the resulting intermediate frequency signal amplified in I. F. amplifier 32 and applied to primary 38 of transformer 31 may be so far removed from the resonant frequency of transformer 31 as to prevent transfer of the intermediate frequency signal to the discriminator tubes 49 and or may even be outside the pass-band of amplifier 32. In this case, terminal 51 will remain at ground po tential in spite of the departure of frequency of oscillator 26 from the desired oscillator frequency, so that the automatic frequency control circuit is unable to vary the voltage applied to heater control tube 8! and thus unable to vary the tuning of oscillator 26 to restore normal operation.

In order to overcome this possibility, a pushbutton reset circuit is provided including storage capacitor 95', relay 96, push-button switch 91, potential source 98 and resistor 99. If no signal indication appears in signal utilization circuit 28, indicating that the frequency of local oscillator 26 may be far removed from the proper local oscillator frequency, push-button switch 91 may be depressed to bring arms ltll and I02 of relay 96 into contact with stationary contacts I03 and [M respectively. This operation charges condenser 95' to substantially the negative potential of source 94', at the same time reducing the potential of the grid of control tube Bl to ground potential, equal to the potential of cathode 94.

In Fig. 3 is shown a graph of the voltage Ec applied to the grid of tube 8| with respect to the cathode, and a simultaneous graph of temperature Ts of the variable tuning strut of ultra high frequency oscillator 26. In this graph of temperature Ts and grid potential Ea against time t, time to is assumed to represent an arbitrary time at which the grid of tube BI is at a substantially constant finite negative potential E0, determined by the position of arm 15 on potentiometer 65, the anode-cathode resistances of tubes BI and 62 being equal and the temperature of the variable tuning strut is held at a corresponding value To by the power applied by oscillator 82 to heater element 86. At the time t1, push-button switch 91 is depressed, closing contacts lfll, I03 and I02, I04 of relay 96, and almost immediately changing the grid potential of tube 8! to cathode potential, thus permitting maximum output of oscillator 82, with the resultant substantially exponential temperature rise indicated by portion ab of curve Ts. Also during the momentary contact of pushbutton switch 91, condenser 95' is charged to substantially the negative potential of D. C. source terminal 94', as described above.

Upon release of push-button switch 91 at time it the contact of arm NH and terminal I03 is broken, and arm I02 is returned to its normal position in contact with terminal I05. The latter operation applies the full negative potential of capacitor 95 to the series resistance circuit, inbinding resistor 99 and resistor I06. A fraction of the potential across condenser 95 is applied through resistor 80 to the grid of series resistor tube 8|. As shown by the abrupt drop 9-41. of curve EG from zero potential to a maximum negative potential at time is, this condenser potential is initially very large, but decreases exponentially as shown at h-lc as the condenser 95 discharges through resistors 99 and I06.

As a result of the initial abrupt application of high negative grid potential to heater oscillator control tube ill, the power delivered by oscillator 82 to heater element 86 is abruptly turned off, with the result that the temperature of strut 90 decreases as shown by portion c-d of curve TB toward a minimum temperature value obtained at time t:, thereafter returning to the value To along a protracted exponential curve d-e generally corresponding to the exponential discharge curve h-Jc of condenser 95.

The frequency of the oscillatory energy produced by tube 26, meanwhile, follows a curve substantially similar to the curve of strut temperature Ts. Thus, the tube 26 is tuned to a maximum oscillation frequency during the period between t1 and t2, while push-button switch 91 is depressed, after which the release of push-button switch 9'! permits the frequency of the oscillator to decrease toward a minimum frequency along a curve similar to that shown for Ts between time is and time ta.

If the desired operating frequency for oscillator 26 is obtained when the tunable strut is heated to the temperature value designated, for example, at R in Fig. 3, then at this point in the downward temperature excursion of the adjustable tuning strut, the intermediate frequency produced in amplifier 32 will reach the proper value for balanced, normal operation of the frequency discriminator 50 employing tubes 49 and Upon further decrease of temperature of the strut and a similar further decrease of the output frequency of oscillator 26 past the tuned frequency of discriminator 56, the frequency discriminator system causes a sharp change of po tential of potentiometer slider I5 in a positive direction as indicated by dotted curve Ea, shown becoming effective at time Ta. The output circult of tubes 6| and 62 is so designed in relation to capacitor 95', resistor 99 and resistor I66 as to permit superior control by the changing potential output of tubes BI and 62 so that the excursion of voltage Ec along curve I63 is abruptly changed as shown in dotted curve Ea, permitting the frequency of oscillation of tube 26 to remain substantially constant at the desired operating frequency as shown by dotted line T5.

If the total tuning range of high frequency oscillator 26, as provided by the strut heating capability of heating oscillator 82 is substantially greater than twice the normal operating frequency of amplifier 32 and the associated frequency discriminator employing tubes 43 and 5i, a. strut temperature lower than that designated at R in Fig. 3 may be found as by positioning switch blade I9 in contact with manual control terminal BI and by variation of the position of slider 92 on potentiometer 93 at which the correct intermediate frequency is produced in amplifier 32. Thus, at temperature R the frequency of local oscillator 26 may be higher than the ultra high oscillation frequency of transmitter 22 by a margin equal to the normal frequency of I. F. amplifier 32; while, on the other hand, with the temperature of strut element 86 reduced to R, the frequency of local oscillator 26 may be lower than the transmitter frequency by the same margin. When the frequency of local oscillator 26 corresponds to temperature R, the amplified discriminator output direct potential provided at arm I5 by tubes GI and 62 is properly sensed for compensation or suppression of any normal variations of frequency of oscillator 26 relative to the carrier frequency of pulsed transmitter 22. On the other hand, when the frequency of oscillator 26 corresponds to strut temperature R, the sensing of the direct potential output of tubes SI and 62 is such as to tend to withhold the temperature of heater element 86 from the temperature designated R. Accordingly, it is essential for best operation of the system that normal strut temperature R be approached from a predetermined temperature extreme, such as shown in Fig. 3, as maximum temperature, T max.

A feature of the push-button reset circuit incorporated in the frequency control system shown resides in the method of forcing heater oscillator 82 to a predetermined operating condition independent of the potential of otentiometer arm 15, to tune the resonant circuit of oscillator 26 to a predetermined frequency extreme, and of thereafter permitting heater element 86 to vary the output frequency of oscillator 26 along a curve of predetermined strut temperature gradient toward a second extreme of the oscillator tuning range, and further permitting the output of the frequency discriminator, amplifier and detector system, including tubes 49, SI, 58, 59, BI and 62, to exercise predominant control over the grid potential of control tube BI when a signal of the proper frequency is provided in I. F. amplifier 32.

From the description of operation of the automatic frequency control system, it will be readily apparent that the direct potential of potentiometer arm 15 is determined by intermediate frequency current pulses rectified differentially through rectifiers 49 and SI, during periods of transmission of transmitter 22 as controlled by pulse generator 2i, During the intervals between transmission pulses, I. F. amplifier 32 and the frequency discriminator 59 comprising tubes 49 and 5| should remain quiescent. During these intervals between the controlling p s from pulse generator 2I, which are usually 1r -n longer than the control pulse duration, it is possible that radio frequency signals received from external sources may excite I. F. amplifier 32, and so cause an incorrect frequency measurement by the frequency discriminator system. In order to guard against such interference, gate control 34 may he provided as a link between the transmitter pulse generator 2| and intermediate frequency amplifier 32 to render the latter inoperative during the intervals between pulses issuing from generator 2|.

Gate control 34 may comprise a pentode tube I2I provided with output coupling resistor I22 connected between the anode I23 of the tube and suppressor grid 36 of amplifier tube 35. A load resistor I24 couples anode I23 to ground, permitting the flow of anode-cathode current through cathode I26 and biasing resistor I21 connected to the negative terminal I28 of a suitable potential source whose other terminal is grounded. A resistor I 29 and a capacitor I3I may be connected in parallel between ground terminal I32 and screen grid I40 of tub I2I to provide screen grid voltage of a proper value. The control grid circuit of tube I2I is coupled by means of resistor I33 and capacitors I34 and I35 to output terminals of pulse generator 2|, so that a high potential negative pulse may be applied to control grid I36 during the radiation from ulse transmitter 22 of a pulse initiated by pulse generator 2I.

When grid I36 is reduced to a high negative potential with respect to cathode I26 by a signal pulse issuing from generator 2|, anode current flow through output resistor I24 is terminated, reducing the potential applied to suppressor grid 36 to ground potential, and thus permitting normal amplification by I. F. amplifier 32. During the intervening periods between pulses from generator 2|, the control grid I36 remains at a moderate or low negative potential with respect to cathode I26, permitting the flow of current through output resistor I24 to anode I23, and thus biasing suppressor grid 36 to a high negative potential, preventing operation of amplifier 32.

Thus, the gate control 34 may be used to guard the intermediate frequency amplifier 32 against passage of an amplified output signal to frequency discriminator 50, and so to suppress interference from signals received from eigternal sources during the intervals between transmitted pulses.

As described above, an automatic frequency control system is provided in which any discrepancy from a desired value of the frequency of oscillations intermittently supplied by a source, is detected in a frequency responsive system comprising a frequency discriminator adapted to produce pulsating output signals indicative of said discrepancy, an alternating-current amplifier adapted to produce an amplified version of the alternating components of said pulsating output signals, and a detector adapted to produce an output potential representative of the time-average of pulsating output signals from the discrimimater.

The combination of the frequency discriminator, the alternating current amplifier, and the detector thus serves as means for producing an amplified output potential sensitive to the frequency of the signals applied to the discriminator. The output potential of the above apparatus is applied to a frequenc control system in such a sense as to suppress discrepancies from a desired frequency of the signals applied to th frequency discriminator.

Gate control 35] insures that the above frequency sensitive means is operative only during the transmitted pulses, thus insuring against interference to the frequency control system from unwanted sources.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for producing electrical oscillations at a frequency of a predetermined difference from the frequency of oscillations of a generator of periodic short-duration pulses of radiofrequency oscillations separated by relatively long time intervals, comprising an adjustable radioirequency oscillator, a mixer coupled to said oscillator to receive energy therefrom and also adapted to receive energy from said generator during said pulses to produce a heterodyne signal, a frequency discriminator coupled to said mixer for providing output potential pulses of magnitude and polarity dependent on the frequency of said heterodyne signal, amplifying means coupled to said discriminator and adapted to amplify alternating current components of said output potential pulses preferentially to direct current components thereof, balanced detector means coupled to said amplifying means and adapted to produce a substantially direct output potential of intensity dependent on the intensity of said discriminator output potential pulses, and tuning means responsive to said balanced detector direct output potential for adjusting the frequency of said oscillator to suppress frequency variations of said heterodyne signal.

2. Apparatus as claimed in claim 1, further including means responsive to said generator for decoupling said frequency discriminator from said mixer during said time intervals of separation of said radio frequency pulses.

3. Apparatus as claimed in claim 1, further including control means coupled to said tuning means for tuning said oscillator to a predetermined frequency during a brief interval and thereafter varying said oscillator frequency at a predetermined rate and in one direction from said predetermined frequency.

4. Electrical apparatus for producing a substantially constant unidirectional output potential of magnitude dependent on the frequency of periodically interrupted electrical oscillations, comprising frequency-sensitive means responsive to said periodically interrupted oscillations to produce a potential similar in wave form to said interrupted oscillations and dependent in amplitude on the frequency of said oscillations, detector means for deriving the envelope of said last-named potential alternating-current amplifying means coupled to said detector means for producing an amplified version of the alternating current component of the output of said detector means, and balanced detector means coupled to said amplifying means to provide an output potential of magnitude dependent on the frequency of said oscillations and variable slowly relative to the period of said interruptions.

5. Automatic electrical frequency control apparatus comprising an adjustable source of recurrent pulses of variable frequency oscillations separated by time intervals of duration different from the duration of said pulses, discriminator means coupled to said source to derive output potential pulses of polarity and magnitude respectively dependent on the direction and extent of variation of the frequency of said oscillations from a predetermined frequency, a detector coupled to said discriminator means to receive said output potential pulses and to produce a direct-current output signal dependent on the alternating components of said pulses, and means responsive to said direct-current output signal and operatively coupled to said source for adjusting the frequency of oscillation of said source in a direction to reduce said variation from said predetermined frequency.

6. Electrical apparatus comprising a source of oscillations periodically alternating between a first amplitude during intervals of a first time duration and a second amplitude during intervals of a second time duration longer than said first duration, rectifier means coupled to said source for producing a signal having alternatingcurrent and direct-current components and characterized by periodic alternation between a first value of said signal during intervals of said first time duration and a second value thereof during intervals of said larger time duration, means for amplifying only said alternating-current components, and further rectifying means having an input circuit coupled to said amplifying means to receive said amplified alternating-current components and an output circuit including filter means, whereby an amplified version of said diroot-current signal component is produced.

7. The method of producing an amplified version of the direct-current component of the envelope of an oscillatory wave characterized by cyclic variations from one value during repetitive time intervals to a second value during longer time intervals alternate with said first time intervals, comprising detecting said wave to produce its envelope, selecting and amplifying the alternating components of said envelope to produce a resultant signal, detecting said resultant signal to produce a signal having a direct-current component, and filtering said direct-current component.

8. Electrical frequency control apparatus comprising an intermittently operating adjustable source of electrical oscillations, means coupled to said source for adjusting the frequency of said oscillations in accordance with an electric signal potential applied thereto, frequency sensitive means having an input circuit for receiving a version of said oscillations and also having an output circuit coupled to said frequency adjusting means for applying thereto a signal potential varying according to variation of the frequency of said oscillations from a desired frequency, whereby said source is adjusted to suppress said variation from said desired frequency, means for coupling said source to said input circuit, and means operative in synchronism with the intermittent operation of said source for rendering said coupling means inoperative during intervals between transmission periods whereby said frequency sensitive means is rendered incapable of receiving extraneous signals during said intervals.

9. Electrical frequency control apparatus comprising an intermittently operating source of ultra-high frequency oscillations, an ultra-high frequency oscillator having a voltage-sensitive frequency controlling element, a mixer coupled to said source and said oscillator for deriving a heterodyne signal of frequency equal to the difference of frequencies of said source and said oscillator and frequency responsive means having an input circuit coupled to said mixer to receive said heterodyne signal and also having an output circuit coupled to said element to vary the frequency of said oscillator in a manner to suppress variations of said heterodyne signal frequency from a desired frequency, and means for blocking said input circuit to the passage of high frequency waves and in synchronism with the intermittent operation of said source, whereby said circuit is in conductive condition only during periods of operation of said source.

10. Electrical apparatus comprising a source of oscillatory energy having an element responsive to an electrical potential to control the frequency of said source over a predetermined range, frequency-responsive means coupled to said source to receive oscillatory energy therefrom and adapted to produce a substantially direct potential having magnitude varying in accordance with deviations of source frequency from a desired frequency, control means having a pair of input terminals and interconnecting said frequency responsive means and said element for varying the potential of said element in response to said direct potential to vary said source frequency in a sense to reduce said direct potential, a condenser connected across said input terminals, manually operable means for short-circuiting said input terminals to apply an extreme potential to said element whereby said source frequency is adjusted to an extreme value and for simultaneously charging said condenser to a predetermined potential, and means operable upon release of said manually operable means for slowly discharging said condenser and for applying the decaying voltage of said condenser across said input terminals to slowly vary said element potential toward an opposite extreme, whereby in the absense of direct potential output from said frequency-responsive means said source frequency is varied from said extreme value toward the opposite extreme, and upon approach of said source frequency near said desired frequency, said direct potential maintains said element potential at a value producing substantially said desired frequency.

11. In a high frequency apparatus having means for transmitting high intensity high frequency electromagnetic energy and means for receiving relatively low-intensity electromagnetic energy reflected from a distant object, the combination comprising a high frequency oscillator having a frequency-controlling element, a first mixer responsive to low-intensity signals and subject to saturation in response to high-intensity signals coupled to said receiving means and said oscillator for deriving a heterodyne signal for producing indications of said object, a second mixer coupled to said oscillator, attenuating means coupling said second mixer and said transmitting means for applying an attenuated portion of said high-intensity energy to said second mixer whereby said second mixer produces a frequency'controlling heterodyne signal whose frequency is dependent upon the frequencies of said high-intensity high-frequency energy and said oscillator, and means responsive to said fre quency-controlling signal and connected to said frequency-controlling element to vary the frequency of said oscillator in accordance with said frequency-controlling signal to suppress variations of the latter from a desired frequency.

12. High frequency apparatus comprising a transmitting channel for high-intensity electromagnetic energy, a receiving channel for receiving relatively low-intensity electromagnetic energy reflected from a distant object, a high frequency oscillator having a frequency-controlling element, a first mixer responsive to low-intensity signals and subject to saturation in response to high-intensity signals coupled to said receiving channel and said oscillator for deriving a first beat signal of frequency equal to the difference between the frequencies of the received energy and said oscillator, utilization apparatus connected in said receiving channel for transforming said beat signal into indications of said object, a second mixer loosely coupled to said transmitting channel and said oscillator whereby said mixer produces a second beat signal whose frequency is dependent upon the frequencies of said high-intensity high-frequency energy and said oscillator, and frequency-responsive means connected to said second mixer and having an output circuit coupled to said frequency-controlling element to vary the frequency of said oscillator in accordance with said second beat signal to suppress variations of said second beat signal frequency from a desired frequency.

CARL C. STOTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,326,737 Andrews Aug. 17, 1943 1,921,168 Royden Aug. 8, 1933 2,113,260 Usselman Apr. 5, 1938 2,358,545 Wendt Sept. 19, 1944 2,287,925 White June 30, 1942 Disclaimer 2,425,013.0arl C. Stotz, Garden City, Y. FREQUENCY CONTROL SYSTEM. Patent dated Aug. 5, 1947. Disclanner filed May 26, 1950, by the assignee, The Sperry Corporation.

Hereb enters this disclaimer to claims 5, 8, 9, 11, and 12 of said patent.

[035ml Gazette July 11, 1950. 

